1. Field of the Invention
The invention relates to an apparatus for placing power control data in a transmission signal, in particular in the form of a CDMA (Code Division Multiple Access) transmission signal.
In mobile radio systems, mobile radio stations MS communicate with a base station BS. Since the number of the mobile stations simultaneously communicating with a base station is high, the transmission power of a transmission signal must be controlled both in the case of the mobile station MS and in the case of the base station BS so that transmission signals of different stations do not interfere with one another. CDMA (Code Division Multiple Access) is a channel access method that is used in cellular mobile radio systems. In CDMA mobile radio systems, a plurality of subscribers, which differ from one another only by an orthogonal subscriber-specific spread signal code applied to the transmission signal, transmit simultaneously in the same frequency band. A separation of the different subscribers is achieved by backspreading by using the respective subscriber-specific orthogonal code. However, received signals are no longer completely orthogonal because of the real mobile radio channel. In order to permit separation of the signals, it is therefore necessary for the various received signals to arrive at the receiver with approximately the same transmission power, since otherwise the desired signals are so strongly disturbed by interference with the transmission signals of other subscribers that separation is no longer possible.
CDMA mobile radio systems therefore require a particularly fast power control of the transmission signal both in the mobile radio station MS and in the base station BS.
The power control of the respective transmitter of a station is performed by power control data that are obtained from the received signal of that station with which the relevant station is communicating. The power of the transmission signal of the base station BS for a specific subscriber is controlled via a power control signal TPC of the mobile station MS. Conversely, the power of the transmission signal of the mobile station MS is controlled by the power control signal TPC transmitted by the base station BS. The generation of the respective control data in a station is performed in this case by evaluating the total signal received by the station by means of a signal-to-interference plus noise ratio estimate, that is to say what is termed an SINR estimate.
FIG. 1 shows a prior art apparatus for inserting power control data.
The transmission data to be transmitted by the station are fed to a coder and multiplexer 40. The coded and multiplexed transmission data are formatted in a formatter into a transmission data word with a plurality of data fields. The transmission data word in this case has a length of one time slot of the mobile radio system. The data fields serve to record information data, pilot data, formatting data and power control data TPC.
The power control data TPC are obtained in a control data generating device 44 by an SINR estimation of a received signal that has been conditioned by a received signal conditioning circuit 46. All data that are to be transmitted within a time slot of the mobile radio system, such as information data, pilot data, formatting data and the power control data TPC, are put together in the formatter 42. The formatting is undertaken, for example, in a DSP processor or microcontroller. The finally formatted transmission data word is transmitted via a signal interoffice trunk, for example a BUS, into an input buffer 48 of a modulator 50. The input buffer 48 usually has a memory capacity that corresponds to the data length of two time slots or two transmission data words. The modulator 50 conditions the transmission data word stored in the input buffer 48 for signal transmission. For this purpose, it uses orthogonal OVSF codes that are stored in a memory 52 to carry out spreading in a CDMA mobile radio system. Furthermore, encryption or scrambling is performed with encryption codes that are generated by an encryption generator V. The transmission signal is subsequently generated by digital-to-analog conversion by a downstream D/A converter 54.
As may be seen from FIG. 1, a delay time interval that corresponds to the memory capacity of the buffer 48 exists between the instant at which the power control data TPC are generated for control on the basis of an interference plus noise ratio estimate, and the instant at which the power control data TPC are conditioned by the moderator 50 for transmission. Particularly in the case of CDMA mobile radio systems that require a particularly fast power control, such a long reaction time until the power control data TPC are transmitted after the SINR evaluation can lead to a substantial worsening of the signal transmission because of interference with transmission signals from other subscribers.
Published European Patent application EP 0 893 889 A2 discloses a method and an apparatus for controlling the transmission power in a CDMA communications system. On the basis of an SINR estimate, a TPC signal is generated and fed to what is termed a level controller, which generates power control data that are then inserted by what is termed the time multiplexer into the power control data field of the buffered transmission data word. The apparatus also has a modulation device for modulating the transmission data word to form a transmission signal with a transmission power corresponding to the power control data.